Microreplicated achromatic lens

ABSTRACT

A microreplicated achromatic lens is disclosed. The article includes a web including first and second opposed surfaces. The first surface includes a first microreplicated structure having a plurality of first features. The second surface includes a second microreplicated structure having a plurality of second features. Opposing first and second features are registered to within 10 micrometers. Corresponding opposed first and second features cooperate to form an achromatic microlens element.

CROSS REFERENCE

This is a continuation-in-part application of U.S. patent applicationSer. No. 10/657,859 now U.S. Pat No. 7,165,959, and U.S. patentapplication Ser. No. 10/658,730 now U.S. Pat. No. 7,224,529, both filedSep. 9, 2003, and incorporated by reference herein.

FIELD

The disclosure relates generally to the continuous casting of materialonto a web, and more specifically to the casting of achromatic lenseshaving a high degree of registration between the patterns cast onopposite sides of the web.

BACKGROUND

In the fabrication of many articles, from the printing of newspapers tothe fabrication of sophisticated electronic and optical devices, it isnecessary to apply some material that is at least temporarily in liquidform to opposite sides of a substrate. It is often the case that thematerial applied to the substrate is applied in a predetermined pattern;in the case of e.g. printing, ink is applied in the pattern of lettersand pictures. It is common in such cases for there to be at least aminimum requirement for registration between the patterns on oppositesides of the substrate.

When the substrate is a discrete article such as a circuit board, theapplicators of a pattern may usually rely on an edge to assist inachieving registration. But when the substrate is a web and it is notpossible to rely on an edge of the substrate to periodically refer to inmaintaining registration, the problem becomes a bit more difficult.Still, even in the case of webs, when the requirement for registrationis not severe, e.g. a drift out of perfect registration of greater than100 micrometers is tolerable, mechanical expedients are known forcontrolling the material application to that extent. The printing art isreplete with devices capable of meeting such a standard.

However, in some products having patterns on opposite sides of asubstrate, a much more accurate registration between the patterns isrequired. In such a case, if the web is not in continuous motion,apparatuses are known that can apply material to such a standard. And ifthe web is in continuous motion, if it is tolerable, as in e.g. sometypes of flexible circuitry, to reset the patterning rolls to within 100micrometers, or even 5 micrometers, of perfect registration once perrevolution of the patterning rolls, the art still gives guidelines abouthow to proceed.

However, in e.g. optical articles such as brightness enhancement films,it is required for the patterns in the optically transparent polymerapplied to opposite sides of a substrate to be out of registration by nomore than a very small tolerance at any point in the tool rotation. Thusfar, the art is silent about how to cast a patterned surface on oppositesides of a web that is in continuous motion so that the patterns arekept continuously, rather than intermittently, in registration within100 micrometers.

SUMMARY

One aspect of the present disclosure is directed to a microreplicatedachromatic lens element. The microreplicated achromatic lens elementincludes a flexible substrate having first and second opposed surfaces.A first coated microreplicated pattern is disposed on the first surfaceand a second coated microreplicated pattern is disposed on the secondsurface. The first and second patterns are registered to within 10micrometers and form an achromatic lens. In some embodiments, aplurality of achromatic lenses are formed on the substrate and have aperiod in a range of 10 to 1000 micrometers and a height in a range of 5to 800 micrometers and lens patterns are registered within 5micrometers, or 3 micrometers or 1 micrometer.

Another aspect of the present disclosure is directed to a method ofmaking a microreplicated achromatic lens element. The method includesproviding a substrate, in web form, having first and second opposedsurfaces, and passing the substrate through a roll to roll castingapparatus to form a plurality of achromatic lens features. Theachromatic lens features include a first microreplicated positive lenson the first surface and a second microreplicated negative lens on thesecond surface. The first and second microreplicated lenses areregistered to within about 10 micrometers.

DEFINITIONS

In the context of this disclosure, “registration,” means the positioningof structures on one surface of the web in a defined relationship toother structures on the opposite side of the same web.

In the context of this disclosure, “web” means a sheet of materialhaving a fixed dimension in one direction and either a predetermined orindeterminate length in the orthogonal direction.

In the context of this disclosure, “continuous registration,” means thatat all times during rotation of first and second patterned rolls thedegree of registration between structures on the rolls is better than aspecified limit.

In the context of this disclosure, “microreplicated” or“microreplication” means the production of a microstructured surfacethrough a process where the structured surface features retain anindividual feature fidelity during manufacture, from product-to-product,that varies no more than about 100 micrometers.

In the context of this disclosure, “achromatic” or “achromat,” lensmeans a lens that is corrected to bring two specified or distinctwavelengths to a common focal point.

BRIEF DESCRIPTION OF THE DRAWINGS

In the several figures of the attached drawing, like parts bear likereference numerals, and:

FIG. 1 illustrates a cross-sectional view of an article made accordingto the present disclosure;

FIG. 2 illustrates a cross-sectional view of a single illustrativeachromatic lens element of FIG. 1;

FIG. 3 illustrates a perspective view of an example embodiment of asystem including a system according to the present disclosure;

FIG. 4 illustrates a close-up view of a portion of the system of FIG. 1according to the present disclosure;

FIG. 5 illustrates another perspective view of the system of FIG. 1according to the present disclosure;

FIG. 6 illustrates a schematic view of an example embodiment of acasting apparatus according to the present disclosure;

FIG. 7 illustrates a close-up view of a section of the casting apparatusof FIG. 4 according to the present disclosure;

FIG. 8 illustrates a schematic view of an example embodiment of a rollmounting arrangement according to the present disclosure;

FIG. 9 illustrates a schematic view of an example embodiment of amounting arrangement for a pair of patterned rolls according to thepresent disclosure;

FIG. 10 illustrates a schematic view of an example embodiment of a motorand roll arrangement according to the present disclosure;

FIG. 11 illustrates a schematic view of an example embodiment of a meansfor controlling the registration between rolls according to the presentdisclosure; and

FIG. 12 illustrates a block diagram of an example embodiment of a methodand apparatus for controlling registration according to the presentdisclosure.

DETAILED DESCRIPTION

Generally, the disclosure of the present disclosure is directed to aflexible substrate coated with microreplicated patterned structures oneach side. The microreplicated articles are registered with respect toone another to a high degree of precision. Preferably, the structures onopposing sides cooperate to give the article achromatic opticalqualities.

Referring to FIG. 1, illustrated is an example embodiment of a two-sidedmicroreplicated achromatic article 14. The article 14 includes a web 20substrate having opposed first and second surfaces 22 and 24. First andsecond surfaces 22 and 24 include first and second microreplicatedstructures 25 and 35, respectively. First microreplicated structure 25includes a plurality of non-planar features 26, which can be spherical,aspherical, cylindrical, or acylindrical, as desired. In the illustratedembodiment aspherical or acylindrical lenses 26 are shown with a pitchor period P of about of 1 millimeter or less, or 1 to 500 micrometers,or 150 to 300 micrometers, or 5 to 50 micrometers, or 8 to 25micrometers, as desired. The non-planar features 26 can form a pluralityof positive lenses 25.

Second microreplicated structure 35 includes a plurality non-planarfeatures 36, which can be spherical, aspherical, cylindrical, oracylindrical, as desired. In the illustrated embodiment spherical orcylindrical lenses 36 are shown with a pitch or period P substantiallymatching or matching the pitch or period P of the aspherical oracylindrical lenses 26. In the illustrated embodiment, first and secondmicroreplicated structures 25 and 35 have the same pitch or period ofrepetition P. The microreplicated achromatic article 14 has a height ofT, which can be any useful height. In some embodiments, the height T isless than 1 millimeter, or 5 to 800 micrometers, or 5 to 50 micrometers,or 200 to 500 micrometers, or 100 to 200 micrometers, as desired.

In the example embodiment shown, opposed microreplicated features 26 and36 cooperate to form a plurality of achromatic lens elements 40. Sincethe performance of each achromatic lens element 40 is a function of theregistration or alignment of the opposed features 26 and 36 forming eachachromatic lens, precision alignment or registration of the lensfeatures is useful. In some embodiments, the opposed features 26 and 36are aligned or in registration to within 5%, or within 2%, or within 1%of the pitch or period P of each achromatic lens element 40. Inillustrative embodiments, the opposed features 26 and 36 are aligned orin registration to within 10 micrometers, or within 5 micrometers, orwithin 3 micrometers, or within 1 micrometer of the each achromatic lenselement 40.

The article 14 described above can be made using an apparatus and methodfor producing precisely aligned microreplicated structures on opposedsurfaces of the web, the apparatus and methods which are described indetail below. In one embodiment the web or substrate 20 includespolyethylene terephthalate (PET), about 25 to 100 micrometers thick.Other web materials can be used, for example, polycarbonate.

In illustrative embodiments, the plurality of aspherical or acylindricalpositive lens features 26 can made on a first patterned roll by castingand curing a first curable liquid onto the first side 22 of the web 20.The first curable liquid can include a photocurable low dispersionmaterial such as, for example a polyacrylate resin solution. Theplurality of spherical or cylindrical negative lens features 36 can bemade on a second patterned roll by casting and curing a second curableliquid onto the second side 24 of the web 20. The second curable liquidcan include a photocurable high dispersion material such as, for examplea polycarbonate or polystyrene resin solution. The aspherical oracylindrical positive lens feature 26 shape and material selection andthe spherical or cylindrical negative lens feature 36 shape and materialselection are determined by one skilled in the art to provide anachromatic lens element 40.

FIG. 2 illustrates a cross-sectional view of a single illustrativeachromatic lens element 40 of FIG. 1. In the illustrated embodiment,light 50 travels through the achromatic lens element 40 and focuses on asingle focal point F. In some embodiments, the achromatic lens element40 has a virtual focal point. In the illustrated embodiment, the lenselement 40 is an achromatic doublet 26, 36 disposed on opposing sides22, 24 of the substrate 20, as described above. In some embodiments, theachromatic lens element 40 is designed to correct longitudinal chromaticaberration with respect to two different wavelengths, e.g., F-line (486nm) and C-line (656 nm). That is, the light beams 50 of the F-line andthe C-line are focused on substantially the same focal point F.

After each respective structure is cast into a pattern, each respectivepattern can be cured using a curing light source such as, for example,an ultraviolet light source. A peel roll can then used to remove themicroreplicated article from the second patterned roll. Optionally, arelease agent or coating can be used to assist removal of the patternedstructures from the patterned tools.

Exemplary process settings used to create articles described above areas follows. A web speed of about 1.0 feet per minute with a web tensioninto and out of casting apparatus of about 2.0 pounds force. A peel rolldraw ratio of about 5% to pull the web off the second patterned tool. Anip pressure of about 4.0 pounds force. A gap between the first andsecond patterned rolls of about 0.010 inches. Resin can be disposed onthe first surface of the web using a dropper coating apparatus and resincan be disposed on the second surface at a rate of about 1.35 ml/min,using a syringe pump.

Curing the first microreplicated structure can be accomplished with anyuseful curing apparatus such as, for example, an Oriel 200-500 W MercuryArc Lamp at maximum power and a Fostec DCR II at maximum power, with allthe components mounted sequentially. Curing the second microreplicatedstructure can also be accomplished with a Spectral Energy UV LightSource, a Fostec DCR II at maximum power, and an RSLI Inc. Light Pump150 MHS, with all the components mounted sequentially.

Generally, the disclosure of the present disclosure can be made by asystem and method, disclosed hereinafter, for producing two-sidedmicroreplicated structures with side-to-side registration of better thanabout 100 micrometers, or better than 50 micrometers, or less than 25micrometers, or less than 10 micrometers, or less than 5 micrometers, orless than 1 micrometer. The system generally includes a first patterningassembly and a second patterning assembly. Each respective assemblycreates a microreplicated pattern on a respective surface of a webhaving a first and a second surface. A first pattern is created on thefirst side of the web and a second pattern is created on the secondsurface of the web.

Each patterning assembly includes means for applying a coating, apatterning member, and a curing member. Typically, patterning assembliesinclude patterned rolls and a support structure for holding and drivingeach roll. Coating means of the first patterning assembly dispenses afirst curable coating material on a first surface of the web. Coatingmeans of the second patterning assembly dispenses a second curablecoating material on a second surface of the web, wherein the secondsurface is opposite the first surface.

After the first coating material is placed on the web, the web passesover a first patterned member, wherein a pattern is created in the firstcoating material. The first coating material is then cured or cooled toform the first pattern. Subsequently, after the second coating materialis placed on the web, the web passes over a second patterned member,wherein a pattern is created in the second coating material. The secondcoating material is then cured to form the second pattern. Typically,each patterned member is a microreplicated tool and each tool typicallyhas a dedicated curing member for curing the material. However, it ispossible to have a single curing member that cures both first and secondpatterned materials. Also, it is possible to place the coatings on thepatterned tools.

The system also includes means for rotating the first and secondpatterned rolls such that their patterns are transferred to oppositesides of the web while it is in continuous motion, and said patterns aremaintained in continuous registration on said opposite sides of the webto better than about 100 micrometers.

An advantage of the present disclosure is that a web having amicroreplicated structure on each opposing surface of the web can bemanufactured by having the microreplicated structure on each side of theweb continuously formed while keeping the microreplicated structures onthe opposing sides registered generally to within 100 micrometer (orbetter as described above.)

Referring now to FIGS. 3-4, an example embodiment of a system 110including a roll to roll casting apparatus 120 is illustrated. In thedepicted casting apparatus 120, a web 122 is provided to the castingapparatus 120 from a main unwind spool (not shown). The exact nature ofweb 122 can vary widely, depending on the product being produced.However, when the casting apparatus 120 is used for the fabrication ofoptical articles it is usually convenient for the web 122 to betranslucent or transparent, to allow curing through the web 122. The web122 is directed around various rollers 126 into the casting apparatus120.

Accurate tension control of the web 122 is beneficial in achievingoptimal results, so the web 122 may be directed over a tension-sensingdevice (not shown). In situations where it is desirable to use a linerweb to protect the web 122, the liner web is typically separated at theunwind spool and directed onto a liner web wind-up spool (not shown).The web 122 can be directed via an idler roll to a dancer roller forprecision tension control. Idler rollers can direct the web 122 to aposition between nip roller 154 and first coating head 156.

A variety of coating methods may be employed. In the illustratedembodiment, first coating head 156 is a die coating head. The web 122then passes between the nip roll 154 and first patterned roll 160. Thefirst patterned roll 160 has a patterned surface 162, and when the web122 passes between the nip roller 154 and the first patterned roll 160the material dispensed onto the web 122 by the first coating head 156 isshaped into a negative of patterned surface 162.

While the web 122 is in contact with the first patterned roll 160,material is dispensed from second coating head 164 onto the othersurface of web 122. In parallel with the discussion above with respectto the first coating head 156, the second coating head 164 is also a diecoating arrangement including a second extruder (not shown) and a secondcoating die (not shown). In some embodiments, the material dispensed bythe first coating head 156 is a composition including a polymerprecursor and intended to be cured to solid polymer with the applicationof curing energy such as, for example, ultraviolet radiation.

Material that has been dispensed onto web 122 by the second coating head164 is then brought into contact with second patterned roll 174 with asecond patterned surface 176. In parallel with the discussion above, insome embodiments, the material dispensed by the second coating head 164is a composition including a polymer precursor and intended to be curedto solid polymer with the application of curing energy such as, forexample, ultraviolet radiation.

At this point, the web 122 has had a pattern applied to both sides. Apeel roll 182 may be present to assist in removal of the web 122 fromsecond patterned roll 174. In some instances, the web tension into andout of the roll to roll casting apparatus is nearly constant.

The web 122 having a two-sided microreplicated pattern is then directedto a wind-up spool (not shown) via various idler rolls. If an interleavefilm is desired to protect web 122, it may be provided from a secondaryunwind spool (not shown) and the web and interleave film are woundtogether on the wind-up spool at an appropriate tension.

Referring to FIGS. 3-5, first and second patterned rolls are coupled tofirst and second motor assemblies 210, 220, respectively. Support forthe motor assemblies 210, 220 is accomplished by mounting assemblies toa frame 230, either directly or indirectly. The motor assemblies 210,220 are coupled to the frame using precision mounting arrangements. Inthe example embodiment shown, first motor assembly 210 is fixedlymounted to frame 230. Second motor assembly 220, which is placed intoposition when web 122 is threaded through the casting apparatus 120, mayneed to be positioned repeatedly and is therefore movable, both in thecross- and machine direction. Movable motor arrangement 220 may becoupled to linear slides 222 to assist in repeated accurate positioning,for example, when switching between patterns on the rolls. Second motorarrangement 220 also includes a second mounting arrangement 225 on thebackside of the frame 230 for positioning the second patterned roll 174side-to-side relative to the first patterned roll 160. In some cases,second mounting arrangement 225 includes linear slides 223 allowingaccurate positioning in the cross machine directions.

Referring to FIG. 6, an example embodiment of a casting apparatus 420for producing a two-sided web 422 with registered microreplicatedstructures on opposing surfaces is illustrated. Assembly includes firstand second coating means 456, 464, a nip roller 454, and first andsecond patterned rolls 460, 474. Web 422 is presented to the firstcoating means 456, in this example a first extrusion die 456. First die456 dispenses a first curable liquid layer coating 470 onto the web 422.First coating 470 is pressed into the first patterned roller 460 bymeans of a nip roller 454, typically a rubber covered roller. While onthe first patterned roll 460, the coating is cured using a curing source480, for example, a lamp, of suitable wavelength light, such as, forexample, an ultraviolet light source.

A second curable liquid layer 481 is coated on the opposite side of theweb 422 using a second side extrusion die 464. The second layer 481 ispressed into the second patterned tool roller 474 and the curing processrepeated for the second coating layer 481. Registration of the twocoating patterns is achieved by maintaining the tool rollers 460, 474 ina precise angular relationship with one another, as will be describedhereinafter.

Referring to FIG. 7, a close-up view of a portion of first and secondpatterned rolls 560, 574 is illustrated. First patterned roll 560 has afirst pattern 562 for forming a microreplicated surface. Second patternroll 574 has a second microreplicated pattern 576. In the exampleembodiment shown, first and second patterns 562, 576 are the samepattern, though the patterns may be different. In the illustratedembodiment, the first pattern 562 and the second pattern 576 are shownas prism structures, however, any single or multiple useful structurescan form the first pattern 562 and the second pattern 576. In anillustrative embodiment, first pattern 562 can be an aspherical negativelens structure and the second pattern 576 can be an aspherical negativelens structure, or vice versa.

As a web 522 passes over the first roll 560, a first curable liquid (notshown) on a first surface 524 is cured by a curing light source 525 neara first region 526 on the first patterned roll 560. A firstmicroreplicated patterned structure 590 is formed on the first side 524of the web 522 as the liquid is cured. The first patterned structure 590is a negative of the pattern 562 on the first patterned roll 560. Afterthe first patterned structure 590 is formed, a second curable liquid 581is dispensed onto a second surface 527 of the web 522. To insure thatthe second liquid 581 is not cured prematurely, the second liquid 581can be isolated from the first curing light 525, by a locating the firstcuring light 525 so that it does not fall on the second liquid 581.Alternatively, shielding means 592 can be placed between the firstcuring light 525 and the second liquid 581. Also, the curing sources canbe located inside their respective patterned rolls where it isimpractical or difficult to cure through the web.

After the first patterned structure 590 is formed, the web 522 continuesalong the first roll 560 until it enters the gap region 575 between thefirst and second patterned rolls 560, 574. The second liquid 581 thenengages the second pattern 576 on the second patterned roll and isshaped into a second microreplicated structure, which is then cured by asecond curing light 535. As the web 522 passes into the gap 575 betweenfirst and second patterned rolls 560, 574, the first patternedstructured 590, which is by this time substantially cured and bonded tothe web 522, restrains the web 522 from slipping while the web 522begins moving into the gap 575 and around the second patterned roller574. This removes web stretching and slippages as a source ofregistration error between the first and second patterned structuresformed on the web.

By supporting the web 522 on the first patterned roll 560 while thesecond liquid 581 comes into contact with the second patterned roll 574,the degree of registration between the first and second microreplicatedstructures 590, 593 formed on opposite sides 524, 527 of the web 522becomes a function of controlling the positional relationship betweenthe surfaces of the first and second patterned rolls 560, 574. TheS-wrap of the web around the first and second patterned rolls 560, 574and between the gap 575 formed by the rolls minimizes effects oftension, web strain changes, temperature, microslip caused by mechanicsof nipping a web, and lateral position control. Typically, the S-wrapmaintains the web 522 in contact with each roll over a wrap angle of 180degrees, though the wrap angle can be more or less depending on theparticular requirements.

To increase the degree of registration between the patterns formed onopposite surfaces of a web, it preferred to have a low-frequency pitchvariation around the mean diameter of each roll. Typically, thepatterned rolls are of the same mean diameter, though this is notrequired. It is within the skill and knowledge of one having ordinaryskill in the art to select the proper roll for any particularapplication.

Referring to FIG. 8, a motor mounting arrangement is illustrated. Amotor 633 for driving a tool or patterned roll 662 is mounted to themachine frame 650 and connected through a coupling 640 to a rotatingshaft 601 of the patterned roller 662. The motor 633 is coupled to aprimary encoder 630. A secondary encoder 651 is coupled to the tool toprovide precise angular registration control of the patterned roll 662.Primary 630 and secondary 651 encoders cooperate to provide control ofthe patterned roll 662 to keep it in registration with a secondpatterned roll, as will be described further hereinafter.

Reduction or elimination of shaft resonance is important as this is asource of registration error allowing pattern position control withinthe specified limits. Using a coupling 640 between the motor 633 andshaft 650 that is larger than general sizing schedules specify will alsoreduce shaft resonance caused by more flexible couplings. Bearingassemblies 660 are located in various locations to provide rotationalsupport for the motor arrangement.

In the example embodiment shown, the tool roller 662 diameter can besmaller than its motor 633 diameter. To accommodate this arrangement,tool rollers may be installed in pairs arranged in mirror image. In FIG.9 two tool rollers assemblies 610 and 710 are installed as mirror imagesin order to be able to bring the two tool rollers 662 and 762 together.Referring also to FIG. 3, the first motor arrangement is typicallyfixedly attached to the frame and the second motor arrangement ispositioned using movable optical quality linear slides.

Tool roller assembly 710 is quite similar to tool roller assembly 610,and includes a motor 733 for driving a tool or patterned roll 762 ismounted to the machine frame 750 and connected through a coupling 740 toa rotating shaft 701 of the patterned roller 762. The motor 733 iscoupled to a primary encoder 730. A secondary encoder 751 is coupled tothe tool to provide precise angular registration control of thepatterned roll 762. Primary 730 and secondary 751 encoders cooperate toprovide control of the patterned roll 762 to keep it in registrationwith a second patterned roll, as will be described further hereinafter.

Reduction or elimination of shaft resonance is important as this is asource of registration error allowing pattern position control withinthe specified limits. Using a coupling 740 between the motor 733 andshaft 750 that is larger than general sizing schedules specify will alsoreduce shaft resonance caused by more flexible couplings. Bearingassemblies 760 are located in various locations to provide rotationalsupport for the motor arrangement.

Because the feature sizes on the microreplicated structures on bothsurfaces of a web are desired to be within fine registration of oneanother, the patterned rolls should be controlled with a high degree ofprecision. Cross-web registration within the limits described herein canbe accomplished by applying the techniques used in controllingmachine-direction registration, as described hereinafter. For example,to achieve about 10 micrometers end-to-end feature placement on a10-inch circumference patterned roller, each roller must be maintainedwithin a rotational accuracy of ±32 arc-seconds per revolution. Controlof registration becomes more difficult as the speed the web travelsthrough the system is increased.

Applicants have built and demonstrated a system having 10-inch circularpatterned rolls that can create a web having patterned features onopposite surfaces of the web that are registered to within 2.5micrometers. Upon reading this disclosure and applying the principlestaught herein, one of ordinary skill in the art will appreciate how toaccomplish the degree of registration for other microreplicatedsurfaces.

Referring to FIG. 10, a schematic of a motor arrangement 800 isillustrated. Motor arrangement 800 includes a motor 810 including aprimary encoder 830 and a drive shaft 820. Drive shaft 820 is coupled toa driven shaft 840 of patterned roll 860 through a coupling 825. Asecondary, or load, encoder 850 is coupled to the driven shaft 840.Using two encoders in the motor arrangement described allows theposition of the patterned roll to be measured more accurately bylocating the measuring device (encoder) 850 near the patterned roll 860,thus reducing or eliminating effects of torque disturbances when themotor arrangement 800 is operating.

Referring to FIG. 11, a schematic of the motor arrangement of FIG. 10,is illustrated as attached to control components. In the exampleapparatus shown in FIGS. 3-5, a similar set-up would control each motorarrangement 210 and 220. Accordingly, motor arrangement 900 includes amotor 910 including a primary encoder 930 and a drive shaft 920. Driveshaft 920 is coupled to a driven shaft 940 of patterned roll 960 througha coupling 930. A secondary, or load, encoder 950 is coupled to thedriven shaft 940.

Motor arrangement 900 communicates with a control arrangement 965 toallow precision control of the patterned roll 960. Control arrangement965 includes a drive module 966 and a program module 975. The programmodule 975 communicates with the drive module 966 via a line 977, forexample, a SERCOS fiber network. The program module 975 is used to inputparameters, such as set points, to the drive module 966. Drive module966 receives input 480 volt, 3-phase power 915, rectifies it to DC, anddistributes it via a power connection 973 to control the motor 910.Motor encoder 912 feeds a position signal to control module 966. Thesecondary encoder 950 on the patterned roll 960 also feeds a positionsignal back to the drive module 966 via to line 971. The drive module966 uses the encoder signals to precisely position the patterned roll960. The control design to achieve the degree of registration isdescribed in detail below.

In the illustrative embodiments shown, each patterned roll is controlledby a dedicated control arrangement. Dedicated control arrangementscooperate to control the registration between first and second patternedrolls. Each drive module communicates with and controls its respectivemotor assembly.

The control arrangement in the system built and demonstrated byApplicants include the following. To drive each of the patterned rolls,a high performance, low cogging torque motor with a high-resolution sineencoder feedback (512 sine cycles×4096 drive interpolation >>2 millionparts per revolution) was used, model MHD090B-035-NG0-UN, available fromBosch-Rexroth (Indramat). Also the system included synchronous motors,model MHD090B-035-NG0-UN, available from Bosch-Rexroth (Indramat), butother types, such as induction motors could also be used.

Each motor was directly coupled (without gearbox or mechanicalreduction) through an extremely stiff bellows coupling, model BK5-300,available from R/W Corporation. Alternate coupling designs could beused, but bellows style generally combines stiffness while providinghigh rotational accuracy. Each coupling was sized so that asubstantially larger coupling was selected than what the typicalmanufacturers specifications would recommend.

Additionally, zero backlash collets or compressive style locking hubsbetween coupling and shafts are preferred. Each roller shaft wasattached to an encoder through a hollow shaft load side encoder, modelRON255C, available from Heidenhain Corp., Schaumburg, Ill. Encoderselection should have the highest accuracy and resolution possible,typically greater than 32 arc-sec accuracy. Applicants' design, 18000sine cycles per revolution were employed, which in conjunction with the4096 bit resolution drive interpolation resulted in excess of 50 millionparts per revolution resolution giving a resolution substantially higherthan accuracy. The load side encoder had an accuracy of +/−2 arc-sec;maximum deviation in the delivered units was less than +/−1 arc-sec.

In some instances, each shaft may be designed to be as large a diameteras possible and as short as possible to maximize stiffness, resulting inthe highest possible resonant frequency. Precision alignment of allrotational components is desired to ensure minimum registration errordue to this source of registration error.

Referring to FIG. 12, in Applicants' system identical position referencecommands were presented to each axis simultaneously through a SERCOSfiber network at a 2 ms update rate. Each axis interpolates the positionreference with a cubic spline, at the position loop update rate of 250microsecond intervals. The interpolation method is not critical, as theconstant velocity results in a simple constant times time interval path.The resolution is critical to eliminate any round off or numericalrepresentation errors. Axis rollover must also addressed. In some cases,it is important that each axis' control cycle is synchronized at thecurrent loop execution rate (62 microsecond intervals).

The top path 1151 is the feed forward section of control. The controlstrategy includes a position loop 1110, a velocity loop 1120, and acurrent loop 1130. The position reference 1111 is differentiated, onceto generate the velocity feed forward terms 1152 and a second time togenerate the acceleration feed forward term 1155. The feed forward path1151 helps performance during line speed changes and dynamic correction.

The position command 1111 is subtracted from current position 1114,generating an error signal 1116. The error 1116 is applied to aproportional controller 1115, generating the velocity command reference1117. The velocity feedback 1167 is subtracted from the command 1117 togenerate the velocity error signal 1123, which is then applied to a PIDcontroller. The velocity feedback 1167 is generated by differentiatingthe motor encoder position signal 1126. Due to differentiation andnumerical resolution limits, a low pass Butterworth filter 1124 isapplied to remove high frequency noise components from the error signal1123. A narrow stop band (notch) filter 1129 is applied at the center ofthe motor-roller resonant frequency. This allows substantially highergains to be applied to the velocity controller 1120. Increasedresolution of the motor encoder also would improve performance. Theexact location of the filters in the control diagram is not critical;either the forward or reverse path are acceptable, although tuningparameters are dependent on the location.

A PID controller could also be used in the position loop, but theadditional phase lag of the integrator makes stabilization moredifficult. The current loop is a traditional PI controller; gains areestablished by the motor parameters. The highest bandwidth current looppossible will allow optimum performance. Also, minimum torque ripple isdesired.

Minimization of external disturbances is important to obtain maximumregistration. This includes motor construction and current loopcommutation as previously discussed, but minimizing mechanicaldisturbances is also important. Examples include extremely smoothtension control in entering and exiting web span, uniform bearing andseal drag, minimizing tension upsets from web peel off from the roller,uniform rubber nip roller. In the current design, a third axis geared tothe tool rolls is provided as a pull roll to assist in removing thecured structure from the tool.

The web material can be any suitable material on which a microreplicatedpatterned structure can be created. Examples of web materials arepolyethylene terephthalate, polymethyl methacrylate, or polycarbonate.The web can also be multi-layered. Since the liquid is typically curedby a curing source on the side opposite that on which the patternedstructure is created, the web material must be at least partiallytranslucent to the curing source used. Examples of curing energy sourcesare infrared radiation, ultraviolet radiation, visible light radiation,microwave, or e-beam. One of ordinary skill in the art will appreciatethat other curing sources can be used, and selection of a particular webmaterial/curing source combination will depend on the particular article(having microreplicated structures in registration) to be created.

An alternative to curing the liquid through the web would be to use atwo part reactive cure, for example, an epoxy, which would be useful forwebs that are difficult to cure through, such as metal web or webshaving a metallic layer. Curing could be accomplished by in-line mixingof components or spraying catalyst on a portion of the patterned roll,which would cure the liquid to form the microreplicated structure whenthe coating and catalyst come into contact.

The liquid from which the microreplicated structures are created can bea curable photopolymerizable material, such as acrylates curable by UVlight. One of ordinary skill in the art will appreciate that othercoating materials can be used, and selection of a material will dependon the particular characteristics desired for the microreplicatedstructures. Similarly, the particular curing method employed is withinthe skill and knowledge of one of ordinary skill in the art. Examples ofcuring methods are reactive curing, thermal curing, or radiation curing.

Examples of coating means that useful for delivering and controllingliquid to the web are, for example, die or knife coating, coupled withany suitable pump such as a syringe or peristaltic pump. One of ordinaryskill in the art will appreciate that other coating means can be used,and selection of a particular means will depend on the particularcharacteristics of the liquid to be delivered to the web.

Various modifications and alterations of the present disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative embodiments set forthherein.

1. A microreplicated achromatic lens comprising: a flexible substratehaving first and second opposed surfaces; a first coated microreplicatedpattern on the first surface, wherein the first coated pattern comprisesa cured first liquid; and a second coated microreplicated pattern on thesecond surface, wherein the second coated pattern comprises a curedsecond liquid, wherein the first and second patterns are registered towithin 10 micrometers and form an achromatic lens, and wherein the firstand second cured liquids have different dispersions.
 2. The achromaticlens of claim 1, wherein the first microreplicated pattern includes aplurality of non-planar lenses.
 3. The achromatic lens of claim 1,wherein the first microreplicated pattern includes a plurality ofaspherical or acylindrical lenses.
 4. The achromatic lens of claim 1,wherein the second microreplicated pattern includes a plurality ofnon-planar lenses.
 5. The achromatic lens of claim 1, wherein the secondmicroreplicated pattern includes a plurality of spherical or cylindricallenses.
 6. The achromatic lens of claim 1, wherein first and secondmicroreplicated patterns cooperate to form a plurality of achromaticlens elements.
 7. The achromatic lens of claim 1, wherein the lens has aperiod in a range of 10 to 1000 micrometers.
 8. The achromatic lens ofclaim 1, wherein the lens has a period in a range of 150 to 300micrometers.
 9. The achromatic lens of claim 1, wherein the lens has aperiod in a range of 5 to 25 micrometers.
 10. The achromatic lens ofclaim 1, wherein the first and second patterns are registered to within3 micrometers.
 11. The achromatic lens of claim 1, wherein the lens hasa height in a range of 5 to 800 micrometers.
 12. The achromatic lens ofclaim 1, wherein the first and second patterns are registered to within1 micrometer.
 13. The achromatic lens of claim 1, wherein the firstmicroreplicated pattern includes a plurality of positive lenses.
 14. Theachromatic lens of claim 13, wherein the second microreplicated patternincludes a plurality of negative lenses.
 15. A method of making anarticle including a plurality of microreplicated achromatic lensfeatures, the method comprising: providing a substrate, in web form,having first and second opposed surfaces; and passing the substratethrough a roll to roll casting apparatus to form a plurality ofachromatic lens features, wherein the achromatic lens features comprisea first microreplicated positive lens on the first surface formed from afirst cured liquid and a second microreplicated negative lens on thesecond surface formed from a second cured liquid, wherein the first andsecond cured liquids have different dispersions, and wherein the firstand second microreplicated lenses are registered to within about 10micrometers.
 16. The method of claim 15, wherein the passing stepcomprises passing the substrate through a roll to roll casting apparatusto form a plurality of achromatic lens features, wherein the achromaticlens features comprise a first microreplicated positive lens on thefirst surface and a second microreplicated negative lens on the secondsurface, and wherein the achromatic lenses have a pitch in a range of 5to 1000 micrometers.
 17. The method of claim 16, wherein the passingstep comprises passing the substrate through a roll to roll castingapparatus to form a plurality of achromatic lens features, wherein theachromatic lens features comprise a first microreplicated positive lenson the first surface and a second microreplicated negative lens on thesecond surface, and wherein the first and second microreplicated lensesare registered to within 3 micrometers.
 18. The method of claim 15,wherein the passing step comprises passing the substrate through a rollto roll casting apparatus to form a plurality of achromatic lensfeatures, wherein the achromatic lens features comprise a firstmicroreplicated positive lens on the first surface and a secondmicroreplicated negative lens on the second surface, and wherein themicroreplicated lenses have a height in a range of 5 to 800 micrometers.19. The method of claim 15, wherein the passing step comprises passingthe substrate through a roll to roll casting apparatus to form aplurality of achromatic lens features, wherein the achromatic lensfeatures comprise a first microreplicated positive lens on the firstsurface and a second microreplicated negative lens on the secondsurface, and wherein the first and second microreplicated lenses areregistered to within 1 micrometer.